Direct Vent Issues:
With the introduction of direct vent equipment, a variety of installation and service issues are being confronted. Primary among these are cross contamination and cold combustion air.
On 12/26/02, we issued a Technical Service Bulletin addressing the problem of cross contamination. This is most often seen with concentric terminations (one penetration venting both flue gases and supplying combustion air to the burner). This is also possible with separate vent and intake terminations if they are located too closely or in the wrong orientation (exhaust vent lower than combustion air intake).
Diagnosis of this problem may be made in two ways: 1) use of an electronic analyzer to measure the percentage of oxygen in the combustion aire, 2) use of an ohmmeter to monitor the cad cell while the burner is running. An electronic analyzer calibrated itself to the amount of oxygen in the atmosphere (approximately 20.9%). Measurements in the combustion intake of less than that indicate cross contamination. Typically, ohm readings will be steady while “seeing” a stable flame. Ohm readings showing wide fluctuations, while the burner is operating, could indicate the presence of cross contamination. This can be confirmed if ohm readings become stable after removal of the combustion air intake from the burner.
With direct vent equipment designed as sealed combustion, the combustion air is delivered directly to the burner. With the extremely cold temperatures we are experiencing this winter, problems have developed with flame stability. As air becomes colder, it gets heavier, denser and hold proportionally more oxygen per pound. Cold air can affect a burner’s ability to maintain flame retention and results in lower CO2 readings. This problem is manifested more so in appliances with very low inputs. Using an ohmmeter to check the cadd cell with verify flame stability.
Proper Appliance Setup:
It is important to follow the appliance manufacturer’s installation instructions and specification when both installing and servicing the equipment. During field visits, we continue to encounter a lack of complete combustion testing.
The only way to verify proper setup is with a combustion test kit, wet or electronic it, does not make a difference. While testing in the field, we have found CO2 readings of 10.5% and lower on new installations. The advent of direct vent equipment and the need to overcome “tight” construction have resulted in an increase in the use of combustion air intake equipment. Cold air may result in a decrease of up to 1% in CO2, further emphasizing the importance of proper burner adjustment.
An equally important part of the testing process is the measurement and control of draft. With traditional heating equipment, we use guiidelines of -0.04″ breech, -0.02″ overfire. Specifications for pressure-fired appliances often have maximum breech draft requirements in some cases as low as -0.03. In fact, actual draft conditions of -0.1 and higher are not unusual today and require particular attention to draft control. Replacement of an appliance in an older home should address the possibility that the existing vent sizing is too large for the new unit and may require alteration. Control of draft may require the use of multiple barometric dampers in some cases.
The combination of high excess air levels (as exhibited by low CO2) and high over fire draft, can cause loss of retention and nuisance lockouts. The use of test equipment will not only assure efficient appliance operation, but will help diagnose and resolve problems in the field.
Loose Cad Cells:
One of the integral components in the cad cell control system is the cad cell assembly itself. The device includes the cad cell eye, a socket assimbly into which the cad cell is installed, and the wiring making the connection to the FF terminals on the control.
We are aware that several of these assemblies have been found in the field which exhibit a loose fit between the cad cell posts and the sockets of the cad cell holder. We advise customers to check the cad cell assimbly to determine how secure the eye is in the socket. If it is loose and readily pulls out with little resistance, we advise complete replacement of the unit.
The operation of the cad cell works on electrical resistance. As light from the flame hits the cadmium sulfide cell, the resistance through it is lowered. Generally, the more light the eye “sees” the lower the resistance and conversely, a lack of light results in high resistance.
Using an ohmmeter, we can both test the operation of the cad cell and monitor the stability of the flame. First, disconnect the cad cell leads from the control. After calibrating the ohmmeter, attach the test leads to the cad cell wires. Start the burner, and once the flame has been established, place a jumper between the FF terminals. While the burner is operating, observe the ohm readins on the meter. (Digital meters may be affected by RF interference if located too closely to the burner causing erratic readings.)
Most control specifications use a maximum of 1500-1600 ohms. Actual readins should be much lower, in the range of 350-900 ohms. Factors influencing the final readins are position of the cad cell assembly, correct burner specifications including nozzle, drawer assembly settings, and combustion test results. Particular attention should be paid to draft and CO2. Excessive over fire draft or too much combustion air exhibited by a low CO2 could cause lifting of the flame off the retention head. This test procedure is simple and will provide a wealth of information about flame stability.
Loose Wire Connections:
Loose wiring connections can temporarily or permanently be interpreted by the control as a false signal, for example, the cad cell scenariou (above). A loose connection can also prohibit a component such as the oil valve, ignitor, or motor from functioning properly. This can cause the burner to malfunction causing the control to recycle or lockout.
When responding to a nuisance lockout call, we ask that you check all wire connections to verify how well they are secured. Often times, replacement of a control has appeared to resolve the problem when in fact redoing a loose wire connection in the process repaired the cause.
We continue to place a high value on input and feedback from our customers. You may forward your questions, suggestions or concerns by calling 1-800-989-2275.